The long-range goal of this research is to understand the neural mechanisms of selective visual attention at the level of the individual neuron and the cortical circuit, and to determine how these mechanisms affect perception and behavior. The conceptual starting point is the observation that the brain is limited in the amount of visual information it can process at any moment in time. For instance, when observers are asked to identify the objects in a briefly presented scene, they become less accurate as the number of objects increases. The proposed research will use a combination of visual psychophysics and single-unit physiology to test two alternative explanations for this capacity limitation, and to investigate the neural mechanisms that enable the brain to select behaviorally relevant stimuli from among irrelevant distracters. Insights from the proposed research are expected to help in understanding, diagnosis and treatment of neuropsychological disorders in which attentional mechanisms fail, such as neglect, Balint's syndrome, ADHD, and attentional aspects of autism.